Shortened turnstile antenna

ABSTRACT

An electrically short turnstile type antenna array which effectively provides vertical or overhead right or left hand circularly polarized coverage in for example satellite communications from a very restricted space volume as characterized particularly in submarine or aircraft antennas. A pair of electrically short dipoles are arranged normal to each other with the axes thereof intersecting at their mid-points. Coaxially arranged about each leg of these dipoles is an openended cylindrical sleeve. The feed to the array is by way of a coaxial cable running to the inside of the coaxial cavity formed by one of the sleeves in an end-feed orientation. A mutual coupling link couples signal energy from the inside of the fed coaxial cavity to the inside of one of the coaxial cavities associated with the other dipole. The entire array is mounted on a support mast less than one-eighth lambda from the horizontal base portion of a tin can-shaped ground plane whose vertical sides are sufficiently high to substantially house the array.

United States Patent [191 Kuecken et al.

[ 1 Jan. 29, 1974 SHORTENED TURNSTILE ANTENNA [75] Inventors: John A. Kuecken, Pittsford, N.Y.;

Michael S. Polgar, Ocean Port, NJ.

[73] Assignee: lntemational Telephone and Telegraph Corporation, Nutley, NJ.

[22] Filed: Apr. 20, 1972 [21] Appl. No.: 246,050

52 05.01 343/792,343/797,343/846, 343/853 51 Int. Cl. ..H01q 21/26 [58] Field of Search... 343/790, 791, 792, 793, 797, 343/846, 853

[56] References Cited Primary ExaminerEli Lieberman Attorney, Agent, or Firm-John T, OHalloran; Menotti l. Lombardi,lr.; Alfred C. Hill 5 7] ABSTRACT An electrically short tumstile type antenna array which effectively provides vertical or overhead right or left hand circularly polarized coverage in for example satellite communications from a very restricted space volume as characterized particularly in submarine or aircraft antennas. A pair of electrically short dipoles are arranged normal to each other with the axes thereof intersecting at their mid-points. Coaxially arranged about each leg of these dipoles is an openended cylindrical sleeve. The feed to the array is by way of a coaxial cable running to the inside of the coaxial cavity formed by one of the sleeves in an endfeed orientation. A mutual coupling link couples signal energy from the inside of the fed coaxial cavity to the inside of one of the coaxial cavities associated with the other dipole. The entire array is mounted on a support mast less than one-eighth A from the horizontal base portion of a tin can-shaped ground plane whose vertical sides are sufficiently high to substantially house'the array.

15 Claims, 7 Drawing Figures PATENTED N B 9 3.789.416 sum 1 or 3 POLAR/2A r/o/v o/AqRAM FEED POINT PATENTEDJAH 29 mm SHEET 2 BF 3 i -aB PAIENIED JAN 2 9 I974 sum 3 0r 3 M. w A R M U Q HYBRID GNAL INPUT on ourpur SHORTENED TURNSTILE ANTENNA BACKGROUND OF THE INVENTION This invention relates to turnstile type antenna arrangements and more particularly to a shortened turnstile antenna arrangement designed .to be operative particularly in the satellite communications bands and in closely packed multi-antenna type configurations.

The antenna installation for working UHF satellites from, for example, a submarine is subject to some rigorous restraints in terms of size. The antenna should have some gain, particularly in the event that-the system noise margin is scanty. However, the volume'usually available in arrangements of this type precludes the installation of a steerable UHF antenna. Accordingly, the next best choice would be a hemispherically directional antenna. Ideally, this antenna would cut off the radiation pattern sharply at the horizon to minimize multipath nulls and fading due to signals bouncing up from the sea. surfacefl-lere again the aperture size restricts the ability to shape the antenna pattern, since the Fourier transform of any rapidly changing antenna pattern requires the existence of high space-harmonic (wide aperture) terms. A'practical antenna would have some coverage below the horizon since the submarine could want to work through satellites on the horizon. The antenna pattern should extend far enough below the level horizon to account for the roll and pitch of the submarine.

The most desirable polarization is circular (left or right), and presumably the entire hemisphere should be filled with the chosen circular polarization. This may not be achieved, however, as can be shown (FIG. 1).

It might at first seem that the use of circular polarization would eliminate the sea bounce and Brewsters angle nulls in the radiation pattern; however, the literature conclusively demonstrates that this advantage does not exist. See Reed, Henry R. and Russell, Carm M. Ultra-High Frequency Propagation Ed. 2, Boston Tech. Pub. Inc., Box ll, Cambridge, Mass. 1966; see pages 219, 235 and FIG. 6-43 on page 246. At low angles, Reed and Russell show that the measured performance of circular polarization is slightly better than horizontal (polarization), but always inferior to vertical polarization. For this reason the use of vertical polarization is recommended for ground-to-air coverage and it is further suggested that an optimum coverage is to be obtained by making the first interference null coincide with the Brewster angle minimum in sea or ground reflection coefficient. Unfortunately, the Brewster angle is too low in the UHF band to permit this optimization since Reed and Russell show the optimum antenna heights to be:

400 MHz 23 Feet 300 MHz 35 Feet 150 MHZ 100 Feet From the preceding arguments it may be concluded that two distinct antennas are probably the best compromise. An upward directed circularly polarized array and a vertically polarized omnidirectional antenna for low angle coverage. We are here concerned with the former primarily. The latter is the subject matter of a copending U.S. Pat. application Ser. No. 217,201 (1. A. Kuecken 3), filed Jan. 12, 1972.

In submarine-to-satellite communications, it is generally the case that space for the antennas is extremely limited. An example of such restricted space requirements may be given by the following, wherein a cylindrical space of say l0 inches (0.254 m) diameter and 6 inches (0.1524 In) may be provided for the circularly polarized radiator, with this volume being for instance situated at the very top position ol'a multi-antenna submarine mast assembly with an unobstructed view of the zenith. For working the satellites, only a limited bandwidth on two separate bands is actually required:

down link 245-255 MHz A I.224-|.l76 m up link 302-3l2 MHz The geometric mean frequency is 276.478 MHz with )t 1.085 m, with the two bands equal to A l.l28 A and A A /Ll28. The'circularly polarized array. for upward coverage must therefore be able to adequately cover both of these bands, i.e. possess the electric characteristic of dual resonant frequencies associative to these satellite frequency bands.

It may be observed that the diameter of the cylindrical volume above-mentioned is on the order of A /4; therefore, the antenna is electrically small and cannot be expected to maintain a well behaved impedance over the desired bandwidth (e.g. 26% bandwidth) without special measures.

SUMMARY OF THE INVENTION The preferred solution is the turnstile array according to the invention.

It is, therefore, an object of this invention to provide a novel antenna arrangement which will meet the exacting space and operative requirements abovedefined.

According to the broader aspects of the invention there is provided a shortened turnstile type antenna arrangement comprising a pair of electrically short dipole antennas arranged normal to each other with the axes thereof intersecting at their mid-points, a pair of openended cylindrical sleeves associated with each of the dipoles and for resonating same, these sleeves having predetermined equal lengths in electrical degrees relative to the intended resonant frequency operation of. the antenna arrangement, and each of the sleeves being arranged to surround a predetermined portion of the associated dipole, each forming thereby a coaxial eavity, and antenna feed means for coupling signal energy to the inside of one coaxial cavity associated with at least one of the dipoles.

A feature of the invention is that the antenna arrangement may be fed in a conventional turnstile antenna configuration as well as by way of the novel feed arrangement described herein.

The novel feed arrangement provides for the feed cableto be coupled to the inside of the coaxial cavity formed by one of the sleeves coaxially arranged around substantially one half of either dipole. A mutual coupling link runs from this fed coaxial cavity to one of the coaxial'cavities of the other dipole antenna. This mutual coupling link is position-adjustable at both ends, and additionally runs inside. the coaxial cavities with which it is associated.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and features of this invention will become more apparent and the invention itself will be best understood by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a polarization diagram illustrating the impossibility of filling an entire hemisphere with right hand circular polarization;

FIGS. 2A and 2B are respectively top and side views of one embodiment of the shortened turnstile antenna arrangement according to the invention;

FIG. 3 is a schematic circuit representation of the antenna array according to FIGS. 2A and 28;

FIG. 4 is an equivalent circuit of the representation,

of FIG. 3; and

FIGS. 5A and 5B are respectively top and side views of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It was stated hereinbefore that an entire hemisphere of left or right hand circular polarization may not be achieved. Referring to FIG. 1, a circle is drawn representing a hemisphere, and pairs of vectors representing the horizontal and vertical components of a right hand circular polarized wave are included therein. Now, in imagining that the arrows can be slid up to the top of the sphere without rotation, it is to be noted that N caneels S and E cancels W. It is thus demonstrated that it is not possible to fill a hemisphere with null-free radiation of any one polarization.

Referring now to the shortened turnstile arrangement according to the invention, an embodiment thereof is represented in FIGS. 2A and 2B in respectively top and side views. A pair of electrically short (approximately 0.2 )t in length) dipole antennas l, 1 are arranged normal to each other with the axes thereof intersecting at their mid-points. Coaxially arranged around each leg of the two dipole antennas, and forming thereby a coaxial cavity, is an open-ended cylindrical sleeve (2a, 2b and 2a, 2b), arranged on the respective dipoles substantially end to end, and having equal length which is determinable by the intended operating frequencies. The cylinders 20, 2b, 2a and 2b are each terminated at the far end with an element 6, i.e. capacitive end loads to resonate (four ends). The entire antenna arrangement is arranged on a support mast 7 (FIG. 2B).

In terms of practical structural considerations, the dipole legs of both antennas l and 1 may, as shown, be comprised of the horizontal portions of separate L- shaped, rigid, metallic pipes or cables brought up through a hollow retaining pipe 7a, and properly isolated electrically from each other. The one cable associated for example with the sleeve 2a would in actuality. be a section of rigid coaxial cable bent in L shape. Each of the other sections forming the remaining legs of the two dipole antennas may also be of rigid coax cable; however, in the embodiment illustrated in FIGS. 2A and 2B there would be no energy directly fed to these three other legs. It is intended rather that the legs asso ciated with sleeves 2b and 2b act as parasitic elements and that the energy to the dipole l is coupled via mutual coupling link 3 from the dipole l tapped at4, with substantially half of the radiated energy of the array being coupled over to dipole 1' according to the actual position of the ends of coupling link 3 at 4 and 10.

One dipole, i.e. dipole 1, is fed via the coaxial cable associated with sleeve 2a at feed point 9 located a predetermined distance W from the one end of the dipole l. The outer conductor of the coaxial feed is electrically' coupled to the end load 6, while the center conductor is coupled to the inside of the coaxial cavity formed by sleeve 2a, and particularly to the inner surface thereof at 9. The mutual signal energy coupling link 3 runs from the portion of dipole antenna 1 at 4 which is a predetermined distance Z from the one end of dipole l, to the dipole antenna 1' at a similarly selected point 10. Mutual couplinglink 3 is shown as a coaxial cable having the outer conductor thereof coupled at the respective ends to the dipole elements 1 and 1' respectively. The center conductor of this coaxial cable is coupled to the inner surface of the respective sleeves 2a and 2a. Mutual coupling link 3 has a predetermined length U depending on intended operating conditions, and physically runs inside the two sleeves (2a, 2a) to which it is coupled. It is particularly intended in this invention to have the coupling points (4 and 10) of link 3 adjustable. This may be accomplished in any suitable manner, such as having a tab coupled to each, wherein the 'tab is accessible outsidethe sleeves 2a and 2a and is permitted to move slidably along a slotcut longitudinally in the sleeve. There may additionally be provided dielectric washer-shaped wafers 5 arranged within the sleeves and around the dipole legs for additional structural stability.

Referring specifically to FIG. 2B there is illustrated in conjunction'with the turnstile type antenna arrangement a tin can-shaped ground plane 8 having a horizontalbase portion 8a and vertical portion 8b. As illustrated, the ground plane houses or surrounds the turnstile antenna arrangement, and is used when it is desired to completely eliminate back and side lobe excitations. The height of the crossed dipole antennas is intended to be less than one-eighth and is represented in FIG. 2 as distance Y.

The electrically short dipole radiator l is resonated at A by means of the shorted choke sleeves 2a and 2b surrounding the rod. These sleeves are in the area of electrical degrees in length at and provide an inductance in series with the capacitive reactance of the antenna formed by the outside of the. sleeve chokes, i.e. a coaxial cavity of less than one-quarter A is provided which appears as an inductive reactance in series with the capacitive reactance and thereby cancels the latter out. The choke also provides a convenient way to pick up nearly any desired input impedance or coupling impedance by providing for the mutual coupling link 3 or the feed cable to be adjustably positioned about the shown coupling points. The tapping done on the coaxial length gives rise to a parallel larger resistance from the series small resistance.

Including the Q raising effect brought about by the small spacing Y (less than one-eighth A) from the ground plane 8, this structure would be expected to behave like a parallel resonant circuit with a Q of about 20. Two such circuits (i.e. the additional consideration of dipole radiator l and its associated sleeves 2a, 2b) may be run as a heavily over-coupled network to obtain the required double response. See FIG. 3 in this regard.

In referring to a standard radio handbook with tables of double tuned circuit response, it may. be found that the separation between the bands will require Cycles off res K Res freq.

K critical coupling coefficient l/Q 0.05 From the curves this very'heavy overcoupling requires 5.0 K actual/K and K actual Thus it may be seen that the tank-to-tank coupling must be much higher than the tank-to-line coupling.

It might seem at first that the connecting line 3 should hex/4 in lengthyhowever, the development of FIG. 4 illustrates the fact that the currents in coupled resonant circuits tend naturally to run in quadrature, thus providing the requisite phasing for circular polarization. Here, the second mesh equation becomes But for resonance, (0 L l/m C and therefore i j w M i R This arrangement may therefore be made to yield a circularly-polarized electrically-short antenna with two well-matched responses at the UP and DOWN-link fre quencies.

The turnstile antenna array according to the invention may also be fed in a somewhat more conventional manner, as illustrated in FIGS. 5A and 58, wherein signal energy is fed to one input of a quadrature hybrid arrangement Q, the other input of which is conventionally terminated, for example in a dummy load. One output of the hybrid circuit is run to the North/South dipole and the other hybrid output, which is 90 in phase relative to the one output running to the North/South dipole, is run to the East/West dipole. Of course, with this feed arrangement, mutual coupling link 3 is no longer required. Power adjustments to the two dipoles 1 and I may be provided by including for example variable attenuators in the output lines of hybrid Q. In this way adjustment of the antenna operating characteristics may be effected remote from the antenna assembly.

As may be seen in FIGS. 5A and 5B, means are provided in the form of outer conductive discs 13 positionadjustable about respective eccentric points 12 for enabling additional tuning capability of the antenna assembly. The metallic discs 13 are arranged snugly parallel to end loads 6, and whenever they are slidably moved about points 12 off of the coincident (minimum capacity) position with end loads 6, the capacitance of the end loads 6 has effectively increased, taking advantage of the large capacitive bulk of the surrounding tin can structure 8. It is within the scope of this invention, moreover, to provide for elements 6 In fact, either arrangement may be readily employed in the embodiment shown in FIGS. 2A and 2B.

In the above there has been described a novel, electrically short turnstile type antenna array which effectively provides vertical right hand circularly polarized coverage in for example satellite communications from a very restricted space volume as characterized particularly in submarines. A pair of electrically short dipoles are arranged normal to each other with the axes thereof intersecting at their mid-points. Coaxially arranged about each leg of these dipoles is an open-ended cylindrical sleeve. The feed to the array is by way of a coaxial line running to the inside of the coaxial cavity formed by one of the sleeves. A mutual coupling link couples signal energy from the inside of the fed coaxial cavity to the inside of one of the coaxial cavities associated with the other dipole. The entire array is mounted on a support mast less than one-eighth from the horizontal base portion of a tin can-shaped ground plane whose vertical sides are sufficiently high to substantially house the array.

While the principles of this invention have been described above in connection with specific apparatus, it is to be understood that this description is made only by way of example and not as a limitation on the scope of the invention as set forth in the objects and features thereof and in the accompanying claims.

themselves to be position-adjustable such that they may be moved along the axis of the respective dipole closer We claim:

1. A shortened turnstile type antenna arrangement, comprising:

a. a pair of electrically short dipole antennas arranged normal to each other with the axes thereof intersecting at their mid-points;

b. a pair of open-ended cylindrical sleeves associated with each of said dipoles and for resonating same, said sleeves having predetermined equal lengths in electrical degrees .relative to the intended operating resonant frequency of the antenna arrangement, and each of said sleeves being arranged to surround a predetermined portion of said associated dipole, forming thereby a coaxial cavity;

c. antenna feed means for coupling signal energy to the inside of one coaxial cavity associated with at least one of said dipoles; and

d. mutual coupling link means for coupling signal energy from said one coaxial cavity associated with said one dipole to one of said coaxial cavities associated with the other of said dipoles.

2. The arrangement according to claim 1 wherein said antenna feed means includes a coaxial feed cable having one of its conducting paths coupled to the inner surface of said sleeve forming said one coaxial cavity with said one dipole a predetermined distance from the one dipole end.

3. The arrangement according to claim 1 wherein said mutual coupling link means includes a coaxial signal energy transmission cable having the conducting paths at the one end thereof coupled respectively to said one dipole a predetermined distance from said one end thereof and inside the coaxial cavity associated therewith and to the inner surface of the sleeve forming said associated coaxial cavity, and the conducting paths at the other end of said signal energy transfer cable being likewise coupled to said other dipole and to the inner surface of one of said coaxial cavity forming sleeves associated therewith.

4. The arrangement according to claim 3 wherein said mutual coupling link means extends within the coaxial cavities associated therewith and is of predetermined length in electrical degrees relative to the intended resonant frequency operation of the antenna arrangement.

5. The arrangementaccording to claim 3 wherein said predetermined distance of the coupling of each end of said mutual coupling link from said associated one end of each said dipole is variable.

6. The arrangement according to claim 3 wherein said predetermined length of each of said sleeves approximates one-half the length of the dipole antenna to which it is associated.

7. The arrangement according to claim 6 wherein said sleeves are end loaded to resonate.

8. The arrangement according to claim 6 wherein said dipoles are comprised of rigid coaxial cable of predetermined length in electrical degrees relative to the intended resonant frequency operation.

9. The arrangement according to claim 3 wherein said antenna feed means includes a coaxial feed cable having one of its conducting paths coupled to the inner surface of said sleeve forming said one coaxial cavity said predetermined distance from the one dipole end.

10. The arrangement according to claim 9 wherein said predetermined distance of said coupled coaxial feed cable from the one end of said one dipole is variable;

11. The arrangement according to claim 3 further comprising a tin can-shaped ground plane in the form of a horizontal conductive surface arranged under the antenna arrangement and having a vertical hollow cylindrical portion extending upward therefrom to house said antenna arrangement.

12. The arrangement according to claim ll wherein the height of said dipoles above said horizontal portion of said ground plane is less than one-eighth A.

13. The arrangement according to claim 3 wherein said antenna feed means includes a quadrature hybrid having the 0 port thereof coupled to one dipole and the port thereof coupled to the other dipole, whereby the signal energy of the latter is 90 in phase relative to the one dipole.

14. The arrangement according to claim 7 wherein said end loads are position-adjustable to provide additional tuning capability.

15. In a tumstile type antenna configuration which includes a pair of dipole antennas arranged normal to each other, each of which is coaxially arranged within a pair of open-ended cylindrical sleeves positioned substantially end-to-end, a feed coupling arrangement comprising a feed cable coupled to one of the dipoles and to the inner surface of one of the coaxial sleeves associated therewith a predetermined distance from the end of said one dipole, so as to provide coupling of the excitation energy to the coaxial cavity formed by said sleeve from the inside, and further including amutual coupling'link cable at one end to said one dipole and said one coaxial sleeve on the inside of the coaxial cavity formed thereby, and likewise coupled at the other end thereof to the other dipole and toone of the sleeves associated therewith inside the coaxial cavity formed thereby. 

1. A shortened turnstile type antenna arrangement, comprising: a. a pair of electrically short dipole antennas arranged normal to each other with the axes thereof intersecting at their midpoints; b. a pair of open-ended cylindrical sleeves associated with each of said dipoles and for resonating same, said sleeves having predetermined equal lengths in electrical degrees relative to the intended operating resonant frequency of the antenna arrangement, and each of said sleeves being arranged to surround a predetermined portion of said associated dipole, forming thereby a coaxial cavity; c. antenna feed means for coupling signal energy to the inside of one coaxial cavity associated with at least one of said dipoles; and d. mutual coupling link means for coupling signal energy from said one coaxial cavity associated with said one dipole to one of said coaxial cavities associated with the other of said dipoles.
 2. The arrangement according to claim 1 wherein said antenna feed means includes a coaxial feed cable having one of its conducting paths coupled to the inner surface of said sleeve forming said one coaxial cavity with said one dipole a predetermined distance from the one dipole end.
 3. The arrangement according to claim 1 wherein said mutual coupling link means includes a coaxial signal energy transmission cable having the conducting paths at the one end thereof coupled respectively to said one dipole a predetermined distance from said one end thereof and inside the coaxial cavity associated therewith and to the inner surface of the sleeve forming said associated coaxial cavity, and the conducting paths at the other end of said signal energy transfer cable being likewise coupled to said other dipole and to the inner surface of one of said coaxial cavity forming sleeves associated therewith.
 4. The arrangement according to claim 3 wherein said mutual coupling link means extends within the coaxial cavities associated therewith and is of predetermined length in electrical degrees relative to the intended resonant frequency operation of the antenna arrangement.
 5. The arrangement according to claim 3 wherein said predetermined distance of the coupling of each end of said mutual coupling link from said associated one end of each said dipole is variable.
 6. The arrangement according to claim 3 wherein said predetermined length of each of said sleeves approximates one-half the length of the dipole antenna to which it is associated.
 7. The arrangement according to claim 6 wherein said sleeves are end loaded to resonate.
 8. The arrangement according to claim 6 wherein said dipoles are comprised of rigid coaxial cable of predetermined length in electrical degrees relative to the intended resonant frequency operation.
 9. The arrangement according to claim 3 wherein said antenna feed means includes a coaxial feed cable having one of its conducting paths coupled to the inner surface of said sleeve forming said one coaxial cavity said predetermined distance from the one dipole end.
 10. The arrangement according to claim 9 wherein said predetermined distance of said coupled coaxial feed cable from the one end of said one dipole is variable.
 11. The arrangement according to claim 3 further comprising a ''''tin can-shaped'''' ground plane in the form of a horizontal conductive surface arranged under the antenna arrangement and having a vertical hollow cylindrical portion extending upward therefrom to house said antenna arrangement.
 12. The arrangement according to claim 11 wherein the height of said dipoles above said horizontal portion of said ground plane is less than one-eighth lambda .
 13. The arrangement according to claim 3 wherein said antenna feed means includes a quadrature hybrid having the 0* port thereof coupled to one dipole and the 90* port thereof coupled to the other dipole, whereby the signal energy of the latter is 90* in phase relative to the one dIpole.
 14. The arrangement according to claim 7 wherein said end loads are position-adjustable to provide additional tuning capability.
 15. In a turnstile type antenna configuration which includes a pair of dipole antennas arranged normal to each other, each of which is coaxially arranged within a pair of open-ended cylindrical sleeves positioned substantially end-to-end, a feed coupling arrangement comprising a feed cable coupled to one of the dipoles and to the inner surface of one of the coaxial sleeves associated therewith a predetermined distance from the end of said one dipole, so as to provide coupling of the excitation energy to the coaxial cavity formed by said sleeve from the inside, and further including a mutual coupling link cable at one end to said one dipole and said one coaxial sleeve on the inside of the coaxial cavity formed thereby, and likewise coupled at the other end thereof to the other dipole and to one of the sleeves associated therewith inside the coaxial cavity formed thereby. 